1. Field of the Invention
The present invention relates to a camera which uses an image blur prevention apparatus such as an image blur correction apparatus.
2. Related Background Art
Cameras are currently automated in all important photographing operations such an exposure determining operation and a focusing operation, thereby allowing even unskilled persons to make photographing errors at remarkably low possibilities.
In the recent years where researches have been made for systems which prevent hand vibrations from being applied to cameras, there remains few causes to induce photographers to erroneous photographing.
Brief description will be made of the system which prevents the hand vibration from being applied to a camera.
The hand vibration which is caused at a photographing time usually has a frequency of 1 Hz to 10 Hz, and a basic concept for enabling to obtain a photograph free from image blur regardless of the hand vibration lies in detecting a camera vibration caused by the hand vibration and displacing a correction lens in accordance with a detected value of the camera vibration. In order to obtain the photograph free from the image blur regardless of the hand vibration, it is therefore necessary first to accurately detect the camera vibration and secondly to correct a variation of an optical axis caused by the hand vibration.
The vibration (camera vibration) can be detected in principle with a camera equipped with a vibration detection apparatus comprising a vibration sensor which detects an acceleration, an angular acceleration, an angular velocity and an angular displacement, and a calculation device which adequately calculates outputs of the vibration sensor in order to correct the camera vibration. The image blur is suppressed by driving correction means which makes a photographing optical axis eccentric on the basis of detection data.
FIG. 28 is a perspective view showing an appearance of a compact camera having a vibration prevention system which has functions to correct a vertical vibration indicated by an arrow 42p and a horizontal vibration indicated by an arrow 42y relative to an optical axis 41.
A camera body 43 comprises a release button 43a, a mode dial (including a main switch) 43b, a retractable strobe 43c and a finder window 43d.
An internal configuration of the camera shown in FIG. 28 is illustrated in FIG. 29, wherein a reference numeral 44 denotes a camera body, a reference numeral 51 denotes correction means, a reference numeral 52 denotes a correction lens, a reference numeral 53 denotes a support frame which corrects the vibrations in directions indicated by the arrows 42p and 42y shown in FIG. 28 by freely driving the correction lens 52 in directions indicated by arrows 58p and 58y in FIG. 29 and will be described later in detail. Reference numerals 45p and 45y denote vibration detection devices such as an accelerometer and an angular velocity sensor which detect vibrations around arrows 46p and 46y respectively.
Outputs from the vibration detection devices 45p and 45y are converted by calculation devices 47p and 47y described later into driving target values for the correction means 51 and input into a coil of the correction means 51 for correcting the vibrations. In addition, a reference numeral 54 denotes a base plate, reference numeral 56p and 56y denote permanent magnets, and reference numeral 510p and 510y denote coils.
FIG. 30 is a block diagram illustrating in detail a configuration of the above described calculation device 47p and 47y, which are similarly configurated, therefore explanation will be made by using only the calculation device 47p.
The calculation device 47p comprises a DC cut filter 48p, a low pass filter 49p, and analog/digital converter circuit (hereinafter referred to as an A/D converter) 410p and a driving device 419p which are enclosed by a chain line, and a camera microcomputer 411 which is enclosed by a dashed line. The camera microcomputer 411 comprises a memory circuit 412p, a differential circuit 413p, a DC cut filter 414p, an integral circuit 415p, a memory circuit 416p, a differential circuit 417p and a PWM duty changing circuit 418p.
A vibrating gyroscope which detects a vibration angular velocity of the camera is used as the vibration detection device 45p, and the vibrating gyroscope is driven when a camera main switch is turned on and starts detecting the vibration angular velocity applied to the camera.
The DC cut off filter 48p configured by an analog circuit cuts off DC bias components which are overlapped with an output signal from the vibration detection device 45p. The DC cut filter 48p is configured by having a characteristic to cut off signal components having frequencies not higher than 0.1 Hz so that these components do not influence on the hand vibration having the frequencies from 1 to 10 Hz applied to the camera. However, the characteristic which cuts off the signal components having frequencies not higher than 0.1 Hz poses a problem that it requires about 10 seconds to cut off the DC components completely after a vibration signal is inputted from the vibration detection device 45p. Accordingly, a time constant of the DC cut filter 48p is shortened (to obtain a characteristic to cut off signal components having frequencies, for example, not higher than 10 Hz) for 0.1 second, for example, after the camera main switch is turned on so that the DC components are cut off in a short time of about 0.1 second and then the time constant is prolonged (to obtain a characteristic to cut off signal components having frequencies not higher than 0.1 Hz) so that a vibration angular velocity signal is not degraded by the DC cut filter 48p.
The low pass filter 49p which is configured by an analog circuit amplifies an output signal from the DC cut filter 48p adequately in accordance with resolution of the A/D converter circuit 410p and cuts off high-frequency noise overlapped with the vibration angle velocity signal from the output signal from the DC cut filter 48p. The noise is cut off from the vibration angular velocity signal so that it will not cause the A/D converter circuit 410p to erroneously sample or read at a step to input the vibration angular velocity signal into the camera microcomputer 411. Furthermore, an output signal from the low pass filter 49p is sampled by the A/D converter circuit 410p and input into the camera microcomputer 411.
Though the DC bias components have been cut off by the DC cut filter 48p, subsequent amplification with the low pass filter 49p allows DC bias components to be overlapped with the vibration angular velocity signal and the DC components must be cut off once again in the camera microcomputer 411.
The memory circuit 412p stores a vibration angular velocity signal which is sampled after 0.2 second, for example, elapses from turning on the camera main switch, and the differential circuit 413p calculates a difference between a stored value and the vibration angular velocity signal so that the DC components are cut off. The DC components can be cut off only roughly by these operations (since not only the DC components but also actual hand vibrations are contained in the vibration angular velocity signal stored after 0.2 second elapses from turning on the camera main switch) and the DC components are cut off sufficiently at a later step with the DC cut filter 414p which is configured by a digital filter. A time constant of this DC cut filter 414p can also be changed in like manner of the analog DC cut filter 48p and gradually prolonged for 0.2 second as measured after 0.2 second elapses from the camera main switch is turned on. Speaking concretely, the DC cut filter 414p has a characteristic as to cut off DC signal components having frequencies not higher than 10 Hz after 0.2 second elapses from turning on the main switch, and then lower the cut off frequency to 5 Hz, 1 Hz, 0.5 Hz and 0.2 Hz at intervals of 50 msec.
However, it may not be desirable to change the time constant while consuming time since a photographer may half depresses the camera main switch (turns on sw1) while the time constant is being changed, whereby the camera may take a photograph upon completing photometry and a distance measurement. In such a case, the camera intercepts the changing of the time constant dependently on photographing conditions. When the photometry indicates a photographing shutter speed of 1/60 at a photographing focal length of 150 mm at which blur prevention may not be so accurate, for example, the camera terminates the changing of the time constant upon obtaining a characteristic to cut off the DC components having frequencies not higher than 0.5 Hz with the DC cut filter 414p (controls the changing of the time constant dependently on a product of the shutter speed multiplied by a photographing focal length). Therefore, the camera is capable of shortening the time required for changing the time constant, thereby making a shutter chance preferential. At a faster shutter speed or a shorter focal length, the camera terminates the changing of the time constant needless to say when the time constant is changed to obtain a characteristic to cut off the DC components having frequencies not higher than 1 Hz with the DC cut filter 414p or at a slower shutter speed or a longer focal length, the camera inhibits photographing until the time constant is changed to the final level.
The integral circuit 415p starts integrating output signals from the DC cut filter 414p when the release button 43a is half depressed (sw1 is turned on), thereby converting an angular velocity signal into an angle signal. When changing of the time constant of the DC cut filter is not completed as described above, however, the integral circuit 415p does not integrate the output signals until the changing of the time constant completes. Though not shown in FIG. 30, the integrated angle signal is adequately amplified dependently on data of a focal length and an object distance at that time, and converted so as to drive correction means 51 in an adequate amount corresponding to a vibration angle. (This correction is required since the photographing optical system is varied and an eccentricity of the optical axis is changed dependently on a driven amount of the correction means 51.)
Upon completely depressing the release button 43a (turning on sw2), the driving of the correction means 51 is started in correspondence to a vibration angle signal but care must be taken to prevent the correction means 51 from abruptly starting a vibration preventive operation. The memory circuit 416p and the differential circuit 417p are provided to prevent the correction means 51 from abruptly starting the vibration preventive operation. When the release button 43a is depressed completely (sw2 is turned on), the memory circuit 416p stores the vibration angle signal from the integral circuit 415p. The differential circuit 417p calculates a difference between the signal from the integral circuit 415p and a signal from the memory circuit 416p. Accordingly, the differential circuit 417p receives two signals which are equal to each other and provides a driving target value signal of 0 to the correction means 51 at a step where the switch SW2 is turned on, thereafter outputting signals successively increasing from 0. (The memory circuit 416p has a role to take an integral signal as an origin at a step where the switch sw2 is turned on.) Accordingly, the correction means 51 is not driven abruptly.
The target value signal from the differential circuit 417p is inputted into the PWM duty changing circuit 418p. Though the correction lens 52 is driven in correspondence to the vibration angle by applying a voltage or a current corresponding to the vibration angle to the coil 510p of the correction means 51 (see FIG. 29), PWM drive is desirable to save electric power to be consumed by the correction means 51 and a driving transistor of the coil.
The PWM duty changing circuit 418p changes a coil driving duty dependently on a target value. When a PWM has a frequency of 20 kHz, for example, the PWM duty changing circuit sets duties "0" and "100" for target values "2048" and "4096" respectively provided from the differential circuit 417p, and determines duties at equal intervals between "0" and "100" in correspondence to target values. The determination of the duties are finely controlled dependently not only on the target values but also photographing conditions of the camera (temperature, camera posture and condition of power supply) so that the vibration is corrected with a high accuracy.
An output from the PWM duty changing circuit 418p is inputted into the driving device 419p which is a known driving device such as a PWM driver and an output from the driving device 419p is applied to the coil 510p (see FIG. 29) of the correction means 51 to correct the vibration. The driving device 419p is turned on in synchronization with the switch sw2 and turned off upon completion of film exposure. So long as the release button 43a is half depressed (sw1 is turned on), the integral circuit 415p continues integration even after the completion of the exposure and the memory circuit 416p stores a new integral output when the switch sw2 is turned on next time.
When the photographer stops half depressing the release button 43a, the integral circuit 415p stops integrating the output from the DC cut filter 414p and resets itself. Resetting means to erase data which has been so far accumulated by integration.
When the main switch is turned off, the vibration detection device 45p is turned off to terminate a vibration prevention sequence.
When the output signal from the integral circuit 415p exceeds a predetermined value, it is judged that the camera has been panned and the time constant of the DC cut filter 414p is changed. For example, a characteristic to cut off DC components having frequencies not higher than 0.2 Hz is changed to a characteristic to cut off DC components having frequencies not higher than 1 Hz and an original time constant is resumed in a predetermined time. An amount of this change of the time constant is controlled dependently on a level of the output from the integral circuit 415p. Speaking concretely, the DC cut filter 414p has a characteristic so as to cut off DC components having frequencies not higher than 0.5 Hz when the output signal exceeds a first threshold value, a characteristic so as to cut off DC components having frequencies not higher than 1 Hz when the output signal exceeds a second threshold value or a characteristic so as to cut off DC components having frequencies not higher than 5 Hz when output signal exceeds a third threshold value.
When the output of the integral circuit 415p is remarkably high, the integral circuit is once reset to prevent a calculation from being saturated (overflowing).
Though the DC cut filter 414p is configured by starting operating after 0.2 second elapses from the main switch is turned on in FIG. 30, this configuration is not limitative and the DC cut filter 414p may be configured by starting operating when the release button 43a is half depressed. In such a case, the integral circuit 415p is operated upon completion of the change of the time constant of the DC cut filter.
Though the integral circuit 415p is also configured by starting operating when the release button 43a is half depressed (when sw1 is turned on), it may be configured by starting operating when the release button 43a is completely depressed (when sw2 is turned on). In such a case, the memory circuit 416p and the differential circuit 417p are unnecessary.
Though the DC cut filter 48p and the low pass filter 49p are disposed in the calculation device 47p in FIG. 30, it is needless to say that these filters may be disposed in the vibration detection device 45p.
FIGS. 31 through 33 are diagrams illustrating the correction means 51 in detail: FIG. 31 is a front view of the correction means 51, FIG. 32A is a side view as seen from a direction indicated by an arrow 32A in FIG. 31, FIG. 32B is a sectional view taken along a 32B--32B line in FIG. 31 and FIG. 33 is a perspective view of the correction means 51.
In FIG. 31, a correction lens 52 is fixed to a support frame 53 (the correction lens 52 comprises two lenses 52a and 52b which are fixed to a support frame 53, and a lens 52c which is fixed to a base plate 54 as shown in FIG. 32B, thereby composing a lens group of a photographing optical system).
A yoke 55 which is made of a ferromagnetic material is attached to the support frame 53, and permanent magnets 56p and 56y which are made of neodymium or the like are adsorbed and fixed (indicated by hidden lines) to a rear surface of the yoke 55 in FIG. 31. Furthermore, three pins 53a which extend radially from the support frame 53 are fitted into elongated holes 54a formed in a side wall 54b of the base plate 54.
The pins 53a are fitted in the elongated holes 54a with no play in a direction along an optical axis 57 of the correction lens 52 but the elongated holes 54a extend in a direction perpendicular to the optical axis 57 as shown in FIGS. 32A and 33, whereby the support frame 53 is freely movable along a plane perpendicular to the optical axis (as indicated by arrows 58p, 58y and 58r) though it is restricted by the base plate 54 in the direction along the optical axis 57. However, the support frame 53 is elastically restricted in each of the directions (58p, 58y and 58r) by a tension spring 59 which is stretched between a hook 53b on the support frame 53 and a hook 54c on the base plate as shown in FIG. 31.
Coils 510p and 510y are disposed on the base plate 54 so as to oppose to the permanent magnets 56p and 56y (indicated by partially hidden lines). The yoke 55, the permanent magnet 56p and the coil 510p are arranged as shown in FIG. 32B (the permanent magnet 56y and the coil 510y are similarly arranged) so that the support frame 53 is driven in the direction indicated by the arrow 58p when a current is supplied to the coil 510p or in the direction indicated by the arrow 58y when a current is supplied to the coil 510y.
A driven amount of the support frame 53 is determined by balance between a spring constant of the tension springs 59 and a thrust which is generated by correlation between the coil 510p, 510y and the permanent magnet 56p, 56y in each directions. That is, an amount of eccentricity of the correction lens 52 can be controlled on the basis of levels of currents supplied to the coils 510p and 510y.
A photographing control described below is conceivable as a hand vibration preventive measure other than the vibration prevention system described above.
A camera determines an exposure time (shutter speed) from luminance of an object, brightness (an F value) of a photographic lens and a film sensitivity, but when the exposure time exceeds a predetermined time, the exposure time is fixed at the predetermined time (the film is not exposed for a time longer than the predetermined time) to prevent an image from being degraded due to the hand vibration and a strobe is lighted to compensate for under-exposure caused by the photographing control. Speaking concretely, the exposure time is fixed at 1/60 second and the strobe is lighted at luminance of an object which requires an exposure time longer than 1/60 second (for example 1/15 second). This photographing control is capable of preventing at a certain degree an image from being degraded due to the hand vibration of a photographer.
However, the photographing control described above does not make it possible of take a favorable image of a photographic scene where strobe light does not reach a main object. When the exposure time is fixed at the predetermined time for a photographic scene where a main object is apart about 7 meters from the camera, for example, the strobe light cannot reach the main object, thereby making the main object under-exposed. Even when the strobe light reaches the main object, the strobe light may not reach a background, thereby making it under-exposed.
Though the hand vibration preventive measure can be improved by equipping a camera which performs the photographing control described above with the vibration prevention system, such a camera cannot cope with the photographing condition where the strobe light does not reach the main object and expose the background adequately.
However, if the strobe light is not used, by using a camera which performs the photographing control described above and is equipped with the vibration prevention system device described above, it is possible to take a photograph with a long exposure time (since it lessens a fear of the hand vibration regardless of the long exposure time), thereby allowing both the main object and the background to be exposed adequately.
Accordingly, it is conceivable that the camera which is equipped with the vibration prevention system is held by hand for photographing a dark object, but the vibration prevention system has a limited capability, thereby posing a problem that camera may be held for too long a time for photographing due to too much reliance on the vibration prevention system, thereby causing photographing to be failed by the hand vibration.